In practical use of a shaft that is mounted in a magnetic bearing, it is known that difficulties occur when the position of the shaft is to be determined in the axial or radial direction in the bearing, particularly when the shaft has an associated magnetic marking whose movement is detected in order to deduce the position or further operating characteristics of the shaft, such as the rotation speed or torque, from the movement of the magnetic marking on the shaft. In this case, particularly in the vicinity of the magnetic bearing, the magnetic field of the magnetic marking is superimposed on the magnetic field of the magnetic bearing, as a result of which it is now virtually impossible to clearly separate the resultant magnetic field, with regard to the contribution made by the magnetic marking.
WO 2006/013092 A1 describes a sensor arrangement for a mounted shaft, wherein the sensor arrangement comprises a signal transmitter a magnetic coding, which is in the form of a magnetized area formed on the body of the shaft, and a receiver which is in the form of a coil, wherein the coil comprises a change in the magnetic field of the magnetic coding. Mechanical stresses in the body of the shaft result in a change in the magnetic field of the magnetic coding because of the magnetostrictive effect, which change is detected by the receiver, and in which case a position of the shaft in the bearing can be deduced indirectly. An arrangement such as this cannot be readily used for a shaft that is mounted in a magnetic bearing, since the magnetic field of the magnetic bearing is superimposed by the change in the magnetization of the magnetic coding. One particular problem is that the magnetic field of the magnetic bearing varies over time, in such a way that the magnetic bearing superimposes a time-dependent signal on the magnetic field measured by the coil. A further disadvantage is that the described measurement arrangement is suitable only for those shafts whose body is composed of a material which is magnetizable, but it is these shafts in particular which are influenced by the magnetic field of the magnetic bearing.
DE 10 2004 025 387 A1 describes a sensor arrangement for a mounted shaft, whose body is composed of an electrically conductive material which is magnetizable. The sensor arrangement comprises a magnet which is arranged at the side on the body, approximately at its center. When an electric current is passed through the body of the shaft, a magnetic field is induced in the body, which is superimposed by the magnetic field of the magnet at the location of the magnet, as a result of which mechanical stresses occur in the body of the shaft at the location of the magnet in the body of the shaft because of the magnetostrictive effect, which mechanical stresses propagate along the body of the shaft and can be detected at one end of the shaft. The distance between the point at which the current pulse was introduced in the shaft and the point at which the magnet is located can be deduced indirectly from the time between the initial current pulse and the occurrence of the mechanical stresses, such that the position of the shaft relative to the magnet is indirectly detectable. The design is complex overall and is virtually impossible to use for a shaft that is mounted in a magnetic bearing, since the magnetic fields of the magnetic bearing continuously cause changing mechanical stresses in the body of the shaft, which are superimposed on the actual measurement variable. In addition, only axial positioning of the shaft can be detected, but not a radial position of the shaft in the bearing.